Significant contributions toward understanding the differentiation of thymocytes into mature, immunocompetent T lymphocytes can come from studies on purified subpopulations of thymocytes. Thymic lymphocytes can be divided into major subpopulations; medullary and cortical thymocytes. To date, studies on medullary thymocytes have used cortisone-resistant thymocytes, and it has not yet been possible to study pure populations of cortical thymocytes. The purpose of the research described here is to characterize purified populations of medullary and cortical thymocytes. We have isolated from the lobster a lectin with sialic acid specificity which appears to agglutinate medullary thymocytes but not cortical thymocytes. Thus, using this lectin, ALgl, preliminary data suggest that we can purify cortical thymocytes as thymocytes which pass through LAgl-coated Sepharose bead columns, and that we can obtain purified medullary thymocytes by specific elution from LAgl-Sepharose columns. Following preparation of these purified subpopulations they will be characterized with respect to surface markers and sialic acid and glycosyltransferase activities. We will then determine the homing characteristics of these cells and their response to T cell mitogens. Finally, we will study whether they can develop into immunologically competent cells. Thus, these studies, by allowing characterization of purified cortical and medullary thymocytes, should provide important information concerning differentiation of thymocytes into mature T cells. Understanding of the development of peripheral T lymphocytes is important for clarification of the normal and abnormal function of T cells during humoral and cell-mediated immunity.